1. Field of the Invention
The present invention relates to a limiting current type oxygen sensor for use in sensing and measuring oxygen concentrations and a method of sensing and measuring oxygen concentrations using the same.
2. Description of the Related Art
A limiting current type oxygen sensor may use an ion conductor composed of, for example, stabilized zirconia of a solid electrolyte with an additive of yttrium oxide (Y2O3), (yttria stabilized zirconia: YSZ), as conventionally known. FIG. 10 is a schematic diagram showing a structure of a conventional limiting current type oxygen sensor. As shown in FIG. 10, a general, conventional limiting current type oxygen sensor 100 includes an ion conductor 101 of a solid electrolyte with an anode 102 and a cathode electrode 103 of porous material formed on both sides thereof to apply a monitor voltage across them. A gas diffusion mechanism, with gas diffusion bores 104, 105 and an internal space 106a to supply a diffusion-rate-determined gas to one electrode 103, is formed in a cap 106, which is attached to the ion conductor.
A heater 107 is provided on the outside of the cap 106 to set the ion conductor 101 at a monitor temperature of several 100° C. The heater 107 is connected to a lead wire 108. The gas diffusion bore 104 is formed through the cap 106 toward the electrode 103 while the gas diffusion bore 105 is formed through the cap along the surface of the electrode 103. Widely used limiting current type oxygen sensors are of the type that do not include one of the gas diffusion bores 105, 106, for example, the gas diffusion bore 105 formed therein.
The limiting current type oxygen sensor 100 is structured such that application of the monitor voltage across the electrodes 102, 103 allows an output current to flow in the ion conductor 101 in proportion to the voltage while the voltage is low. The limiting current type oxygen sensor 100 also has a characteristic that the output current saturates in time as the monitor voltage is elevated. The output current in the saturation region is called a limiting current. The intensity of the limiting current has a relationship to the oxygen concentration. Therefore, the limiting current type oxygen sensor 100 makes it possible to sense and measure an oxygen concentration from a limiting current value obtained in accordance with the monitor voltage.
The current flowing in the ion conductor 101 of the limiting current type oxygen sensor 100 is based on the migration of oxygen ions and has a current value that depends on the voltage and the temperature. Therefore, the limiting current type oxygen sensor 100 is set at a monitor temperature of around 400-500° C. and voltage-driven. Usually, the monitor temperature is set by providing the heater 107 on the cap 106 or a body portion of the limiting current type oxygen sensor 100, and energizing it.
Such a limiting current type oxygen sensor 100 is driven through a method of applying the monitor voltage across the electrodes 102, 103 while the heater 107 is always energized in many cases (see Patent Document 1: Japanese Patent No. 3373741, for example). In order to increase the output current value on sensing and measuring a lower oxygen concentration, the conventional limiting current type oxygen sensor 100 thus configured is provided with a gas diffusion mechanism that includes not only the gas diffusion bore 104 but also the gas diffusion bore 105 formed through the cap 106.
In the limiting current type oxygen sensor 100 provided with the gas diffusion mechanism structured to include bore portions such as the gas diffusion bores 104, 105 formed through the cap 106, the gas diffusion mechanism is configured to satisfy the condition of the following expression (1) to measure an oxygen concentration:
[EXPRESSION 1]
                    IL        =                                                            -                4                            ⁢              FDSP                        RTL                    ⁢                      ln            ⁡                          (                              1                -                                                      P                                          O                      ⁢                                                                                          ⁢                      2                                                        P                                            )                                                          (        1        )            as a relation among the Faraday constant (F); a diffusion coefficient (D); a bore area (S) of the bore portion; a total gas pressure (P); a gas constant (R); a temperature (T); a bore length (L) of the bore portion; a partial oxygen pressure (Po2); and an output current value (IL).
In such a limiting current type oxygen sensor, the gas diffusion mechanism may be configured such that the distance in the internal space between the electrode and an inner surface opposed thereto, that is, the thickness of the internal space 106a in the axial direction of the gas diffusion bore 104 is made smaller than the bore diameter of the gas diffusion bore 104. In this case, the output current value exhibits the following characteristic illustrated in FIG. 11. FIG. 11 is a graph showing a characteristic of voltage (V)-current (I) in the conventional limiting current type oxygen sensor of such the type. As shown in FIG. 11, if the thickness of the internal space in the gas diffusion mechanism of the limiting current type oxygen sensor is smaller than the bore diameter of the gas diffusion bore, the relation between the bias voltage (Vs) and the output current (Is) in the sensor has the characteristic represented by solid line 111. Namely, if the thickness of the internal space is smaller, oxygen molecules diffused through the gas diffusion bore into the internal space are diffusion-rate-determined also in the internal space before reaching the end of the cathode electrode. Therefore, a flat zone 112 on the solid line 111 exhibits a larger vertical tilt (a non constant value) as a characteristic. Therefore, in the conventional limiting current type oxygen sensor, the limiting current value in accordance with the monitor voltage is represented by such a characteristic, and the oxygen concentration is sensed and measured based on this value. Thus, conventional limiting current type oxygen sensors with a smaller thickness of the internal space have the characteristic of the flat zone with the larger tilt in a voltage-current relation. Consequently, ripples, fluctuations or variations in the monitor voltage deteriorate the accuracy of sensing the limiting current value and make it difficult to obtain the limiting current of an accurate value.
In the above-described conventional limiting current type oxygen sensor 100, sensing and measuring a lower oxygen concentration requires the formation of a new gas diffusion bore 105 through the cap 106 in addition to the gas diffusion bore 104. This requires a larger number of process steps in the process of producing the limiting current type oxygen sensor 100 and complicates the sensor structure itself, resulting in an increased production cost.
Further, in the limiting current type oxygen sensor 100 the new gas diffusion bore 105 is formed through the cap along the surface of the electrode 103 (the electrode surface) on the ion conductor 101. Therefore, compared with the type that includes no gas diffusion bore 105 formed therein, a larger variation in production accuracy may be caused easily, depending on the process accuracy. As a result, the variation in tilt characteristic is caused and the measurement accuracy is not stabilized.
In addition, the bore diameter of the gas diffusion bore 104 may be enlarged, instead of the gas diffusion bore 105 as in the conventional limiting current type oxygen sensor described in the above Patent Document 1, and no gas diffusion bore 105 is provided. In such a sensor, within a limiting current region with oxygen concentrations of 1% or below, the limiting current value can not satisfy the condition of the above expression (1). Accordingly, it is not possible to sense and measure an accurate oxygen concentration.